Melt-spun polypropylene fine-grade nanofibrous web

ABSTRACT

The present invention is directed toward a to fine-grade stand-alone nanoweb and nanofibrous membrane comprising a nanofiber network with a number average nanofiber diameter less than 200 nm and the mean flow pore size less than 1000 nm that yield the selective barrier medium with a superior balance of flow versus barrier properties.

This application claims the benefit of priority of U.S. Provisional Application No. 61/893,961 filed Oct. 22, 2013, which is incorporated herein by reference in it's entirety.

FIELD OF THE INVENTION

This invention relates to melt-spun polypropylene fine-grade nanofibrous web comprising a nanofiber network with a number average nanofiber diameter less than 200 nm and the mean flow pore size less than 1000 nm.

BACKGROUND

The increased surface to volume ratio afforded by nanofibers has significant influences on a broad range of applications. In particular, in filter performance, which is based on producing the highest flow rate while trapping and retaining the finest particles without blocking the filter, nanofibers have improved interception and inertial impaction efficiencies and result in slip flow at the fiber surface, affording better performance at a given pressure drop. Consequently, nanofibers as a coating layer on substrate or laminated with a substrate are currently incorporated into filters in air, liquid and automotive applications.

Polymer nanofibers can be produced from solution-based electrospinning or electroblowing, however they have very high processing cost, limited throughputs and low productivity. Melt blowing nanofiber processes that randomly lay down fibers do not provide adequate uniformity at sufficiently high throughputs for most end use applications. The resulting nanofibers are often laid on substrate layer of coarse fiber nonwoven or microfiber nonwoven to construct multiple layers. A problem with melt-blown polypropylene nanofibers or small microfibers, exposed on the top of the web, they are very fragile and are easily crushed by normal handling or contact with some object. Also, the multilayer nature of such webs increases their thickness and weight, and also introduces some complexity in manufacture. Centrifugal spun nanofiber process has demonstrated the lower manufacturing cost in massive nanoweb production.

U.S. Pat. No. 8,277,711 B2 to DuPont discloses a nozzle-less centrifugal melt spin process through rotational thin film fibrillation. The nanofibers with number average diameter less than about 500 nm have been claimed and shown in the examples spun from polypropylene and polyethylene resins. In practice, the operation window is very narrow for making the uniform nanofibers due to the requirement of uniform and smooth thin film flow on the inner surface of the spinning disk, which requires the right rheological properties of polymer and the right combination of the temperature, the rotating speed and melt feeding rate. Otherwise, there would not have a uniform and smooth thin film flow on the inner surface of the spinning disk. As results, the instability of the thin film flow and variation of the thickness in the thin film will cause the formation of larger fibers mixed with the nanofibers.

The nanofibers made from the process of U.S. Pat. No. 8,277,711 B2 can be laid on a belt collector to form uniform web media using the process of WO 2013/096672, in which the complicate air flow management needs to be implemented. Otherwise, the uniform web cannot be laid down because of the swirling and the twisting of fiber stream due to the “tornado”-like effect under the high speed rotating disk. U.S. Pat. No. 8,231,378 B2 to University of Texas (later the FibeRio Technology Corporation) discloses a centrifugal nanofiber spinning from rotating spinnerets with nozzles), such as, syringes, micro-mesh pores or non-syinge gaps with a typical openings of diameter sizes of 0.01-0.80 mm. The microfibers with the number average diameter of one micron or larger and the nanofibers have been shown. The nanofiber with number average diameter less than about 300 nm has been disclosed. In general, the centrifugal spinning through nozzles has much less throughput due to the capillary flow through the nozzle orifices and the melt die swell at the nozzle exit. For the current state of the art, only very low basis weight of thin layer nanofibers can be deposited on scrim when the polypropylene nanofiber is spun from a melt. The polypropylene about 600nm has been reported with the mixture of fibers with defects, especially the powders and and the“spatters”. The PP web has very low strength and is difficult to handle without scrim due to the thermal degeradation.

What is needed is the improvement of centrifugal melt spun nanofiber process to make to make a fine-grade nanofibrous web.

SUMMARY OF THE INVENTION

The present invention is directed toward a melt-spun polypropylene fine-grade nanofibrous web comprising a nanofiber network with a number average nanofiber diameter of less than about 200 nm and the mean flow pore size of less than about 1000 nm.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a low magnification SEM image and FIG. 1B is a high magnification SEM image of the web structure in the present invention.

FIG. 2 is an illustration of the apparatus using a spinning disk based on the process of U.S. Pat. No. 8,277,711 B2 with the improvements according to the present invention.

FIG. 3 is an illustration of the spin disk with the stand-alone web collector for improving process of U.S. Pat. No. 8,277,711 B2 according to the present invention.

FIG. 4A is the graphical form of the number average of nanofiber diameter distribution of Example 1 in the present invention. FIG. 4B is the tabular form of the number average of nanofiber diameter distribution of Example 1 in the present invention.

FIG. 5 is the pore size distribution of Example 1 in the present invention.

FIG. 6 is the thermogravimetric analysis (TGA) data of web sample of Example 1 and the polymer resins pellets used in making Example 1.

FIG. 7 is the molecular weight (Mw) data of web of Example 1 and the polymer resins pellets used in making Example 1 measured by using high temperature size exclusion chromatography (SEC).

FIG. 8 is the Differential Scanning calorimeter (DSC) thermal analysis data of web sample of Example 1 and the polymer resins pellets used in making Example 1.

FIGS. 9A and 9B show the SEM images of Comparative Example 1 at 250× and 10,000× magnification, respectively.

FIG. 10 is the pore size distribution of Comparative Example 1.

FIGS. 11A and 11B show the SEM images of Comparative Example 2 at 250× and 10,000× magnification, respectively.

FIG. 12 is the pore size distribution of Comparative Example 2.

DETAILED DESCRIPTION Definitions

The term “web” as used herein refers to layer of a network of fibers commonly made into a nonwoven.

The term “nonwoven” as used herein refers to a web of a multitude of essentially randomly oriented fibers where no overall repeating structure can be discerned by the naked eye in the arrangement of fibers. The fibers can be bonded to each other, or can be unbounded and entangled to impart strength and integrity to the web. The fibers can be staple fibers or continuous fibers, and can comprise a single material or a multitude of materials, either as a combination of different fibers or as a combination of similar fibers each comprising of different materials.

The term “nanofibrous web” as used herein refers to a web constructed predominantly of nanofibers. “Predominantly” means that greater than 50% of the fibers in the web are nanofibers.

The term “nanofibers” as used herein refers to fibers having a number average diameter less than about 1000 nm. In the case of non-round cross-sectional nanofibers, the term “diameter” as used herein refers to the greatest cross-sectional dimension.

The term “microfibers” as used herein refers to fibers having a number average diameter from about 1.0 μm to about 3.0 μm

The term “coarse fibers” as used herein refers to fibers having a number average diameter greater than about 3.0 μm.

The term “coarse-grade nanofibrous web” as used herein refers to the nanofibrous web having the mean flow pore size greater than about 5.0 μm.

The term “intermediate-grade nanofibrous web” as used herein refers to the nanofibrous web having the mean flow pore size greater than about 1.0 μm and smaller than 5.0 μm.

The term “fine-grade nanofibrous web” as used herein refers to the nanofibrous web having the mean flow pore size smaller than about 1.0 μm.

The term “stand-alone” as used herein refers to the nanofibrous web is a single layer, self-contained and without any substrate.

The term “centrifugal spinning process” as used herein refers to any process in which fibers are formed by ejection from a rotating member.

The term “rotating member” as used herein refers to a spinning device that propels or distributes a material from which fibrils or fibers are formed by centrifugal force, whether or not another means such as air is used to aid in such propulsion.

The term “concave” as used herein refers to an inner surface of a rotating member that can be curved in cross-section, such as hemispherical, have the cross-section of an ellipse, a hyperbola, a parabola or a frustoconical, or the like.

The term “spin disk” as used herein refers to a rotating member that has a disk shape with a concave, frustoconical or flat open inner surface.

The term “fibril” as used herein refers to an elongated structure that may be formed as a precursor to fine fibers that form when the fibrils are attenuated. Fibrils are formed at a discharge point of the rotating member. The discharge point may be an edge, serrations or an orifice through which fluid is extruded to form fibers.

The term “nozzle-free” as used herein refers to the fibril or fibers that are not from a nozzle-type spinning orifices, including nozzles on a rotating member.

The term “charged” as used herein refers to an object in the process that has a net electric charge, positive or negative polarity, relative to uncharged objects or those objects with no net electric charge.

The term “spinning fluid” as used herein refers to a thermoplastic polymer in either melt or solution form that is able to flow and be formed into fibers.

The term “discharge point” as used herein refers to a location on a spinning member from which fibrils or fibers are ejected. The discharge point may, for example, be an edge, or an orifice through which fibrils are extruded.

The term “serration” as used herein refers to a saw-like appearance or a row of sharp or tooth-like projections. A serrated cutting edge has many small points of contact with the material being cut.

The term “tornado-like” as used herein refers to a violently rotating column of fibers that is in contact with both the surface of the collector and a cumulonimbus cloud of the swirling fiber bundles.

The term “essentially” as used herein refers to that if a parameter is held “essentially” at a certain value, then changes in the numerical value that describes the parameter away from that value that do not affect the functioning of the invention are to be considered within the scope of the description of the parameter.

The present invention is directed toward to melt-spun polypropylene fine-grade stand-alone nanoweb and nanofibrous membrane comprising a nanofiber network with a number average nanofiber diameter around or less than 200 nm and the mean flow pore size less than 1000 nm, the SEM images as shown as in FIG. 1, the number average of nanofiber diameter distribution as shown as in FIGS. 4A and 4B and the pore size distribution as shown as in FIG. 5.

In principle, the nonwoven web can be made using the centrifugal melt spinning process as disclosed in U.S. Pat. No. 8,277,711 B2. The nanofiber formation is through uniform thin film fibrillation. The melt flow spread on the inner surface of the spin disk to form a thin film. The film fibrillation occurs at the edge of spinning disk and forms thin threads. These thin threads are further stretched into fibers by centrifugal force. For a given polymer, nanofibers are formed from a uniform stable thin film fibrillation in U.S. Pat. No. 8,277,711 B2. The operation parameters of fiber spinning are temperatures, melt feeding rate and disk rotating speed. In practice of U.S. Pat. No. 8,277,711 B2, the fully pure nanofibers can only be made from the uniform and smooth thin film flow on the inner surface of the spinning disk, which requires the right rheological properties of polymer and the right combination of the temperature, the rotating speed and melt feeding rate. However, the surface of the rotating polymer thin film on the inner surface on the open-end spinning disk would be cooling down due to reaction with the cold air brought in by the high speed rotating. In practice, the heating to the spinning disk would be to the higher temperature in order to have the right melt viscosity and the uniform thin film flow. Therefore, there was a potential thermal degradation if the temperature was set too high. The present invention is about to address this problem. A thermal shield on top of the spinning disk is designed to minimize the reduction of the surface temperature of the rotating polymer thin film. With the thermal shield on top of the spinning disk will lower the disk heating temperature to minimize or to eliminate the thermal degradation.

Considering FIG. 2 for spinning disk 205 mounted on a high speed rotating hollow shaft 200, fibers 210 are shown exiting the discharge points at the edge of the spinning disk. A protecting shield 206 with the same diameter as the spinning disk is mounted on top of the spinning disk as a thermal protecting shield for melt spinning in order to prevent the heating lost to the inner surface of the spinning disk; as an air protecting shield for solution spinning to prevent the rapidly solvent evaporation from the thin film flow on the inner surface of the spinning disk.

The protecting shield is placed to contact to the serrations on the edge of the rotating disk to form an enclosed serrations. The enclosed serrations on the edge of the rotating disk suppress the instability of the thin film flow and variation of the thickness at the edge of the spinning disk.

A stationary shield 208 for the spinning disk is mounted on a stationary shaft through the rotating hollow shaft at the bottom of the spinning disk to protect the thermal lost, and to prevent the swirling and the twisting of fiber stream due to the “tornado”-like effect under the high speed rotating disk for the uniform web laydown.

A stretching zone surrounding the edge of the rotating disk is indicated in the dash line rectangle area. The stretching zone temperature is established by the gentle air comes from the combination of three heating air streams. One is from the gentle heating air 202 above the spinning disk; another is from a stream of gentle heating air 209 coming from a stationary hot air tube within the rotating hollow shaft 200, through the gap between the bottom of the spinning disk and the stationary shield to reach the stretching zone; the other gentle heating air is a downward flow 201. The stretching zone temperature is designed and implemented to keep the threads in molten state to maximize the stretching or elongation by the centrifugal force. The stretching zone diameter is about 1.5 times the diameter of the spin disk. The stretching zone temperature is the key element to make the nanofibers. For polypropylene in Example, the stretching zone temperature is optimized around 180° C. by the gentle heating air for the better nanofiber spinning and for the fibers to take electrostatic charging as an option.

The nanofibers are deposited on the surface of a horizontal scrim belt collector or a vertical tubular scrim belt collector using the web laying process of WO 2013/096672, then a roll of the web is wind-up as a stand-alone web roll off from the collection belt. Typically, fibers do not flow in a controlled fashion towards the collector and do not deposit evenly on the collector. The improved process of WO 2013/096672 with the stationary shield under the spinning disk is used in the present invention. The stationary shield prevents the “tornado”-like affect under the high speed rotating disk, therefore, the swirling and the twisting of fiber stream are eliminated in the present invention. A charged ring 203 is optional with needle assembly or a ring saw with sharp teeth is mounted on the top of stretching zone air heating ring for applying the electrostatic charge to fibrils and fibers 210 being ejected from a spinning disk.

Considering FIG. 3 for the fibers laydown on a belt collector to form nanofibrous web, 301 is the spin pack shown in FIG. 2. The nanofibrous web 300 is laid on a vacuum box web laydown collector 310 may be placed under the whole spin pack. The collector may have a perforate surface. Vacuum is applied to collector with the highest vacuum strength at the corners and the edges of the collector and the vacuum strength gradually reduce moving away from the corners and the edges of the collector to the center of the collector where the vacuum strength is zero. The fibers were collected on a circling belt 302, driven by 303, with 304 as a tension adjusting roll, 305 is a supporting roll for the stand-alone nanofibrous web, and the stand-alone web is sent through a pair of nips 306 and onto a wind-up roll, 307, and is taken up.

The present invention is directed toward a melt-spun polypropylene fine-grade nanofibrous web comprising a nanofiber network with a number average nanofiber diameter of less than about 200 nm and the mean flow pore size of less than about 1000 nm.

The nanofiber network has a fiber diameter both mean and median of less than about 200 nm and the individual nanofibers have a fiber diameter in the range of a minimum of about 10 nm to a maximum of about 1000 nm.

The nanofibrous web has: (a) less than about 5% Mw reduction of the nanofibrous web as compared to the polymer used for making the nanofibrous web, (b) essentially the same thermal weight loss as compared to the polymer used for making the nanofibrous web as measured by TGA, and (c) higher crystallinity of the nanofibrous web as compared to the polymer used for making the nanofibrous web.

Test Methods

High-Speed Video Image: In order to visualize the filming and fiber spinning, high-speed video image has been used for observing the spinning of poly(ethylene oxide) (PEO) in water solutions. Weight percent solutions ranging between 0% and 12% of 300,000 Mw PEO, purchased from Sigma-Aldrich, were prepared in deionized water. A Harvard apparatus PHD2000 Infusion syringe pump was used to control the flow rate of solution to a rotating geometry spinning between 1,000 and 30,000 RPM. Flow rates examined range between 0.01 to 50.00 mL/min. Two Photon FASTCAM SA5 model 1300K-M3 high speed video cameras with Canon 100 mm Macro lenses were used to capture the images included in this case with one camera positioned parallel and one camera positioned perpendicular to the spinning geometry. The camera and lens settings were chosen to maximize clarity at 7,000 fps, shudder speeds ranging between 0.37 and 4.64 ps, and apertures between f2.8 and f32.

Thermal analysis: In order to study the thermal degradation and crystallinity, thermal analysis was conducted using TA Instruments a Q2000 series differential scanning calorimeter (DSC) and a Q500 series thermo gravimetric analyzer (TGA). DSC samples underwent a standard heating, cooling, re-heating cycle from room temperature to 250° C. at 10° C./min under nitrogen. TGA samples underwent a standard ramp heat from room temperature to 900° C. at 10° C./min under nitrogen. TA Instruments Universal Analysis 2000 was used to analyze thermal data.

The percent crystallinity of samples was determined using the accepted value for the enthalpy of fusion for 100% crystalline polypropylene equaling 207 J/g. (REFERENCE: A van der Wal, J. J Mulder, R. J Gaymans. Fracture of polypropylene: The effect of crystallinity. Polymer, Volume 39, Issue 22, October 1998, Pages 5477-5481)

Measurement of Molecular Weight: Molecular weight for polyolefin resins was measured by using high temperature size exclusion chromatography (SEC). This method includes the use of multi-angle light scattering and viscosity detectors in trichlorobenzene (TCB) at 150° C. The instruments used include a Polymer Laboratories PL220 liquid chromatograph instrument, with solvent delivery and autoinjector, and a Wyatt Technologies Dawn HELEOS multi-angle light scattering detector (MALS). The Polymer Laboratories SEC includes an internal differential viscometer and differential refractometer. Four Polymer Laboratories mixed B SEC columns were used for the separations. The sample injection volume was 200 microliters with a flow rate of 0.5 mL/min. The sample compartment, columns, internal detectors, transfer line, and Wyatt MALS were held at a controlled temperature between 150 and 160° C. depending on the polymer. After the solution passes through the columns within the Polymer Laboratories SEC, the flow was directed out of the instrument and through a heated transfer line to the Wyatt MALS before being returned back to the Polymer Laboratories SEC. The data recovered from the instrumentation was analyzed using Wyatt Technologies Astra software. The concentration was calculated using a do/dc of 0.092 for polyolefin in TCB. Molecular weights were calculated from the light scattering intensities rather than elution time, and are not relative to standards. In order to ensure instrument performance and accuracy, available NIST polyethylene standards are periodically analyzed.

Measurement of Web Strength: Tensile strength and elongation of nanofibrous web samples were measured using an INSTRON tensile tester model 1122, according to ASTM D5035-11, “Standard Test Method for Breaking Force and Elongation of Textile Fabrics (Strip Method)” with modified sample dimensions and strain rate. Gauge length of each sample is 2 inches with 0.5 in. width. Crosshead speed is 1 inch/min (a constant strain rate of 50% min⁻¹). Samples are tested in the “Machine Direction” (MD) as well as in the “Transverse Direction” (TD). A minimum of 3 specimens are tested to obtain the mean value for tensile strength or elongation.

SEM: Scanning Electron Microscope (SEM) image was used dominantly in nanofiber characterization because it delivers superb image clarity at high magnification and has become the industry standard for measuring nanofiber diameter. The differences of nanofiber morphology in high magnification SEM images with ×5,000 or ×10,000 of nanofibrous webs produced from different nanofiber processes are difficult to be distinguished beside the fiber diameter. In order to reveal the fiber morphology in different levels of details, the SEM images were taken at ×25, ×100, ×250, ×500, ×1,000, ×2,500, ×5,000 and ×10,000.

Mean Flow Pore Size was measured according to ASTM E 1294-89, “Standard Test Method for Pore Size Characteristics of Membrane Filters Using Automated Liquid Porosimeter.” Individual samples of different size (8, 20 or 30 mm diameter) were wetted with the low surface tension fluid as described above and placed in a holder, and a differential pressure of air was applied and the fluid removed from the sample. The differential pressure at which wet flow is equal to one-half the dry flow (flow without wetting solvent) is used to calculate the mean flow pore size using supplied software. Mean flow pore size was reported in μm.

Bubble Point was measured according to ASTM F316, “Standard Test Methods for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test.” Individual samples (8, 20 or 30 mm diameter) were wetted with the low surface tension fluid as described above. After placing the sample in the holder, differential pressure (air) is applied and the fluid was removed from the sample. The bubble point was the first open pore after the compressed air pressure is applied to the sample sheet and is calculated using vendor supplied software.

Pore Size Uniformity Index: The uniformity index (UI) for the pore size is defined as the ratio of the difference in bubble point diameter and the minimum pore size to the difference in the bubble point and mean flow pore.

${UI}_{pore} = \frac{{BP} - {Min}}{{BP} - {MFP}}$

The closer this ratio is to the value of 2, and then the pore distribution is a Gaussian distribution. If the Uniformity Index is very much larger than 2, the nanofibrous structure is dominated by pores whose diameters are much bigger than the mean flow pore. If the Uniformity Index (UI) much lower than 2, then the more structure is dominated by pores which have pore diameters lower than the mean flow pore diameter. There will still be a significant number of large pores in the tail end of the distribution.

EXAMPLES Example 1

Continuous fibers were made by a spin disk with the enclosed serrations and the stationary shield using an apparatus as illustrated in FIG. 3, from a polypropylene (PP) homopolymer, Metocene MF650Y from LyondellBasell. It has Mw=75,381 g/mol, melt flow rate=1800 g/10 min (230° C./2.16 kg), and zero shear viscosity of 9.07 Pa.S at 200° C. The temperature of the spinning melt from the melt transfer line was set to 240° C. The temperature of spin disk edge was about 200° C. The stretching zone heating air was set at 250° C. The stretching zone air through the gap between the disk and the stationary shield was set at 200° C. with the air flow rate of 50 SCFH. The downward shaping air was set at 150° C. The shaping air flow was set at 50 SCFH. The rotation speed of the spin disk was set to a constant 10,000 rpm.

The fiber size was measured from an image using scanning electron microscopy (SEM) as shown as in FIG. 1 and the distribution of the number average diameter of the nanofibers is shown in FIG. 5. Example 1 has a fiber diameter mean and median for the total fibers measured of 217.31 nm and 193.85 nm from total counts of 973 individual nanofibers in the range of the minimum of 64.12 nm to the maximum of 872.47 nm, respectively. The PMI measurement result shows that the nanofibrous web has a mean flow pore (MFP)=504.1 nm, MO=465.6 nm, Min=197.7 nm and Max (BP)=3442.2 nm. 51 MFP-M0|=38 nm, UI=1.104.

FIG. 6 shows the almost identical TGA measurement of the nanofibrous web of Example 1 and the polymer resin pellets used in making the web. FIG. 7 shows the macromolecules weight measurement of the nanofibrous webs of Example 1 and the polymer resin pellets used in making the web. There is small reduction of macromolecules weight in the nanofibrous webs of Example 1 comparing to the polymer resin pellets used in making the web. FIG. 8 shows the crystallinity of the nanofibrous web is higher than the polymer resin used for making nanofibers from the DSC measurement. Overall, the measurements show that the thermal degradation has been reduced to minimum.

Comparative Example 1

Continuous fibers were made by an open-end spin disk using the process of U.S. Pat. No. 8,277,711 B2, from the same polypropylene (PP) homopolymer used in Example 1. A PRISM extruder with a gear pump was used to deliver the polymer melt to the rotating spin disk through melt transfer line. The temperature of the spinning melt from the melt transfer line was set to 200° C. The temperature of spin disk edge was to be about 240° C. The stretching zone heating air was set at 200° C. The downward shaping air was set at 150° C. The shaping air flow was set at 15.0 SCFM. The rotation speed of the spin disk was set to a constant 10,000 rpm.

The fiber size was measured from an image using scanning electron microscopy (SEM) as shown as in FIGS. 9A and 9B. Comparative Example 1 has a fiber diameter mean and median for the total fibers measured of 685 nm and 433 nm from total counts of 583 individual nanofibers in the range of the minimum of 126 nm to the maximum of 8460 nm.

Comparative Example 2

Continuous fibers were made by an open-end spin disk using the process of U.S. Pat. No. 8,277,711 B2, from the same polypropylene (PP) homopolymer used in Example 1. The temperature of the spinning melt from the melt transfer line to the rotating spin disk was set to 200° C. The temperature of spin disk edge was about 200° C. The stretching zone heating air was set at 180° C. The downward shaping air was set at 150° C.

The shaping air flow was set at 50.0 SCFH. The rotation speed of the spin disk was set to a constant 10,000 rpm.

The fiber size was measured from an image using scanning electron microscopy (SEM) as shown as in FIGS. 11A and 11B. Comparative Example 2 has a fiber diameter mean and median for the total fibers measured of 935 nm and 670 nm from total counts of 431 individual fibers in the range of the minimum of 172 nm to the maximum of 17,052 nm. There are about 83.88% nanofibers, 14.92% of microfibers and 1.2% coarse fibers. 

What is claimed is:
 1. A melt-spun polypropylene fine-grade nanofibrous web comprising a nanofiber network with a number average nanofiber diameter of less than about 200 nm and the mean flow pore size of less than about 1000 nm.
 2. The nanofibrous web of claim 1, wherein the nanofiber network has a fiber diameter both mean and median of less than about 200 nm and the individual nanofibers have a fiber diameter in the range of a minimum of about 10 nm to a maximum of about 1000 nm.
 3. The nanofibrous web of claim 1, wherein the nanofibrous web has: (a) less than about 5% Mw reduction of the nanofibrous web as compared to the polymer used for making the nanofibrous web; (b) essentially the same thermal weight loss as compared to the polymer used for making the nanofibrous web as measured by TGA; and (c) higher crystallinity of the nanofibrous web as compared to the polymer used for making the nanofibrous web. 